Developments in the optical fiber communications field have been rapid. However, the technology still is undergoing major shifts in direction. For example, earlier generation fiber systems were designed to operate at wavelengths of about 0.8 .mu.m, and current systems operate at 1.3 .mu.m. Now there is growing interest in systems having an operating wavelength of about 155 .mu.m to take advantage of the loss window that exists in silica-based optical fiber in that wavelength region. Another example of a major shift which is driven by demand for higher bandwidths is that from multimode to single mode fibers.
Although desired for their large bandwidth capabilities and small size, light-transmitting optical fibers are mechanically fragile, exhibiting brittle fracture under tensile loading and degraded light transmission when the fiber is bent. A cable for use in a duct must be capable of withstanding tensile loads applied when the cable is pulled into the duct and stresses caused by bends which may be frequent in loop plant in urban areas. As a result, cable structures have been developed to protect mechanically the optical fibers.
Cable structures which have been developed for optical fibers include loose tube, ribbon and stranded cables. For a description of loose tube cables, see, for example, D. Lawrence and P. Bark "Recent Developments in Mini-Unit Cable" published at pp. 301-307 of the Proceedings of the 32nd International Wire and Cable Symposium, 1983. See also U.S. Pat. No. 4,153,332. Ribbon cable comprises a core of one or more ribbons with each including a plurality of optical fibers disposed generally in a planar array. The core is surrounded by a loose-fitting plastic inner tubular jacket and an outer jacket reinforced with strength members. Another optical communications cable which is suitable for use in duct systems is disclosed in U.S. Pat. No. 4,241,979 which issued on Dec. 30, 1980 in the names of P. F. Gagen and M. R. Santana. A bedding layer, about which strength members are wrapped helically, is added between plastic extruded inner and outer jackets to control the extent to which the strength members are encapsulated by the outer jacket. The cable includes two separate layers of strength members, which are wrapped helically in opposite directions. Under a sustained tensile load, these two layers of strength members produce equal but oppositely directed torques about the cable to insure the absence of twisting. In another type of optical communications cable, a plurality of optical fibers are enclosed in an extruded plastic tube to form a unit and a plurality of these tubed units are enclosed in a common extruded plastic tube which is enclosed in a sheath system. The optical fibers which are enclosed in each unit tube are stranded together about a central strength member.
Generally, optical fiber cables of the prior art, such as ribbon and stranded and loose tube, suffer from the disadvantage of having the ribbons, the stranded units or the tubes manufactured on a separate line. In stranded cable, for example, a plurality of units which priorly have been enclosed individually in tubes and stranded are fed into a line which applied the common tube and the outer jacket. Each of the units must be made separately on another line and inventoried until a plurality of them can be associated together in the common tube. Because the ribbon or tubed core is generally stranded with a predetermined lay, its manufacture and the assembly of the ribbons or tubes into the core involve the use of relatively heavy rotating apparatus which may be undesirable from a manufacturing standpoint.
Also, in an optical fiber cable, perturbations along the axes of the optical fibers, which are referred to as microbends, can cause optical loss by allowing power to escape through the cladding. The degradation in transmission which results from this type of bending is known as microbending loss. For a discussion of microbending loss, see S. E. Miller et al, Optical Fiber Telecommunications, Academic Press, New York, (1979) pp. 158-161; H. G. Unger, Planar Optical Waveguides and Fibers, Clarendon Press, Oxford, Chapter 6, pp. 552-648; and D. Marcuse "Microdeformation Losses of Single Mode Fiber", Applied Optics, vol. 23 no. 7, Apr. 1, 1984, pp. 1082-1091. This problem may occur, for example, when a waterblocking filling material is introduced into the cable in order to prevent the incursion of water. Typically, waterblocking materials of the prior art do not yield under strains experienced when the cable is made or handled. This prevents the movement of the optical fibers within the cable and the fibers buckle because they contact, with a relatively small periodicity, a surface of the unyielding filling material. This is overcome somewhat by stranding the fibers which allows the fibers under stress to form new helices to avoid microbending losses. However, as is well known, stranding requires the use of a lower line speed.
These problems have been addressed by an optical fiber cable which is disclosed in application Ser. No. 721,533 which was filed on Apr. 10, 1985 in the names of C. H. Gartside III, A. D. Panusska, and P. D. Patel (now abandoned). It includes a plurality of optical fibers which are assembled together into units without intended stranding. All the units are disposed in a common plastic tube instead of having each unit disposed within an associated individual tube. The common tube may be filled with a grease-like composition having a relatively low critical yield stress as disclosed in application Ser. No. 697,054 filed on Jan. 31, 1985 in the names of C. H. Gartside, III et al (now U.S. Pat. No. 4,701,016).
With sophisticated methods of manufacture for this last-described cable and with the fibers being enclosed in coatings, these microbending losses are not discernible at room temperature. The coatings absorb perturbations at room temperature, allowing the fiber to remain substantially unperturbed. However, at relatively low temperatures, that is in the range of -40.degree. F., the coating material experiences thermal changes thereby causing the optical fiber axes in the cable to bend. Because of the properties of the coating material and of the cable, the coating material may only partially absorb these perturbations and some are transferred through to the optical fibers.
While the aforementioned cable has overcome many problems, improvement in the consistency of its performance at relatively low temperatures has been desired. At relatively low temperatures in the range of about -40.degree. F., the performance of cables made in accordance with the abovedescribed invention should be substantially microbending insensitive. What is needed is an optical fiber cable which is compact and which inhibits the introduction of undue streses that could lead to microbending losses in the optical fibers over commonly accepted temperature ranges.